Program Director/Principal Investigator (Last, First, Middle): Mendelsohn, Cathy Lee Project Summary The urothelium is an epithelial barrier that prevents exchange of water and toxic substances between the blood and urinary tract. It is composed of basal cells, Intermediate cells and a luminal layer of superficial cells. Superficial cells are specialized for synthesis and transport of uroplakins, a family of proteins that assemble into the crystalline apical plaque that is the functional urothelial barrier. Superficial cells have a number of interesting features; they are enormous, generally binucleated and are polyploid. The urothelium is one of the most quiescent epithelia in the body, but quickly regenerates in response to acute damage, for example from urinary tract infection. Chronic injury however, from repeated exposure to toxins such as cyclophosphamide, can lead to barrier compromise, inflammation and disease such as bladder pain syndrome, a life altering condition for which there is no cure at present. These proposed studies build on previous findings from our lab that identify novel urothelial progenitors that are important for development and regeneration. In the present application, we will use lineage analysis and an organ culture together with live imaging to visualize the behavior of adult wild type and mutant urothelial progenitors. We will pursue preliminary studies suggesting that Basal cells, which are unipotent during acute injury, acquire the potential to produce Intermediate and Superficial cell daughters after chronic injury. We identify Pparg, a nuclear receptor best known for its role in adipogenesis, as a potential target of Retinoids, and we show that Pparg is critical for formation and regeneration of Superficial cells, and also regulates basal cell homeostasis, preventing squamous differentiation. Besides its critical role in human health, the urothelium is a also source of cells that give rise to different types of bladder cancers. Identification of urothelial progenitors and the signaling pathways that control them will be important for developing therapeutic strategies for regeneration and repair, and for understanding how urothelial differentiation can go wrong.